Communication systems that operate at radio frequencies (RF) require small and low cost bandpass filters. These bandpass filters are used to select transmit or receive signals within a certain band width BW at a specified frequency and to reject signals at frequencies outside the band width. Some examples include global positioning systems GPS, mobile telecommunication systems: Global Systems for Mobile Communications GSM, personal communication service PCS, the Universal Mobile Telecommunications System UMTS, Long Term Evolution Technology LTE, data transfer units: Bluetooth, Wireless Local Area Network WLAN, satellite broadcasting and future traffic control communications. They also include other high frequency systems for high rate data transmission/acquisition in air and space vehicles.
Bandpass filters for RF signal filtering are fabricated using different technologies: (a) ceramic filters based on dielectric resonators, (b) filters using surface acoustic wave resonators (SAW), and (c) filters using thin film bulk acoustic wave resonators (FBAR). For mobile communication systems such as handsets, although the power capability required for the RF filters is only 5 W or less, the size and cost requirements are rather critical. The main properties of piezoelectric materials for filters are acoustic wave propagation velocity and coupling coefficient. The velocity determines the resonant frequency with electrode pitch and the coupling coefficients affect the band width. For FBARs, the main properties are velocity of acoustic waves and thickness of active piezoelectric layer.
FIG. 1A shows a schematic cross-sectional diagram of a prior art film bulk acoustic resonator FBAR (100a) on a substrate (110) with a substrate thickness (110t). It comprises an air cavity (115) having an air cavity depth (115t), a bottom electrode (140) with a bottom electrode thickness (140t), a piezoelectric layer (150) having a piezoelectric layer thickness (150t) and a top electrode (190) with a top electrode thickness (190t). The purpose of the air cavity (115) is to prevent the acoustic waves from getting into the substrate. Therefore, the acoustic energy is confined in the piezoelectric layer (150) with a minimum loss into the substrate (110). The resonant frequency of the FBAR (100a) is mainly determined by the piezoelectric layer thickness (150t). On the other hand, due to the presence of the air cavity (115), the dissipation of heat generated in the piezoelectric layer (150), the bottom metal electrode layer (140) and the top metal electrode layer (190) to the substrate (110) of the FBAR is limited. Hence, in order to prevent circuits instability due to excessive heating of the piezoelectric layer (150), the operating power of systems involving FBARs with an air cavity (115) cannot be too high. In the prior art FBAR shown in FIG. 1A, the piezoelectric layer (150) is responsible for the interactions between the acoustic waves and the RF electric field to be applied between the top electrode layer (190) and bottom electrode layer (140). Because the piezoelectric layer (150) has a constant thickness (150t), the acoustic waves resonant frequency are fixed.
In order to improve dissipation of heat from the piezoelectric layer, another structure of FBAR has been adopted and used in RF filters. FIG. 1B shows a schematic cross-sectional diagram of another prior art FBAR (100b) named solidly mounted bulk acoustic resonator (SMBAR or SMR). The SMBAR (100b) is deposited on the substrate (110) with a substrate thickness (110t), having a thin film reflector stack (120) having alternating low impedance layers (120-L1, 120-L2, 120-L3) and high impedance layers (120-H1, 120-H2) with a total thin film reflector stack thickness (120Tt), a bottom electrode (140) with a bottom electrode thickness (140t), a piezoelectric layer (150) with a piezoelectric layer thickness (150t), a top electrode (190) with a top electrode thickness (190t). The resonant frequency fo is determined by the velocity v of the acoustic waves and the piezoelectric layer thickness (150t). To confine the acoustic wave energy in the piezoelectric layers and to minimize loss to the substrate (110), low impedance layer thickness (120-Lt) and high impedance layer thickness (120-Ht) are selected to be λ/4, here λ is the wavelength of the acoustic wave in the respective layer. As there is no air gap between the piezoelectric layer (150) and the substrate (110), the dissipation rate of heat generated in the piezoelectric layer (150) and electrode layers (140, 190) to the substrate (110) is greater than that of the FBAR (100a) with an air cavity (115) as shown in FIG. 1a. In the prior art FBAR (100b), the piezoelectric layer (150) is the active layer responsible for the interactions between RF electric field to be applied between the top electrode layer (190) and the bottom electrode layer (140) and the acoustic waves. Because the thickness of the piezoelectric layer is constant, the resonant frequency of acoustic waves is fixed. Therefore, once a filter involving FBARs has been fabricated, the central frequency fo of transmission and bandwidth BW are fixed by the geometry and materials used.
Due to the large number of bands used in the mobile handsets in different regions and countries, and even in the same country, a practical handset needs to have an RF front end covering several frequency bands. A true world phone will need to have about 40 bands, each with a transmit band and receive band. As each RF filter has only one central frequency of resonant and a fixed bandwidth, therefore, such a true world phone will need to have 80 filters for the front end. Due to the resource limitations, some designers design mobile phone handsets to cover 5 to 10 bands for selected regions or countries. Even with this reduced number of bands, the number of RF filters currently required is still large: from 10 to 20 units. Therefore, there is a strong need to reduce the dimensions and cost of the RF filters. It would be ideal to reduce the number of filters by having tunable RF filters, each to cover at least two frequency bands. If this is successful, the number of filters can be reduced in the mobile handsets and many other microwave and wireless systems. Thus, it would be critical to develop an RF filter which can cover as many bands or frequency ranges as possible so that the size and power consumption of RF front ends in a mobile phone handset and microwave systems can be reduced.